1. Reading the history of a person's web browser to see where they have been.

2. Pop-up advertising can be activated by active browser sniffing via spyware. The spyware senses that a person has searched for a term or that a term appears on a web page visited by the user and then a pop-up ad appears that is vaguely related to that term.

3. People that surf via unsecured wireless are likely to have their surfing sessions sniffed also.